Transparent image-recording elements are primarily intended for viewing by transmitted light, for example, observing a projected image from an overhead projector. In a typical application, the viewable image is obtained by applying liquid ink dots to an ink-receptive layer using equipment such as ink jet printers involving either monochrome or multicolor recording.
It is known that the ink-receptive layers in transparent image-recording elements must meet stringent requirements including, an ability to be readily wetted so there is no "puddling", i.e., coalescence of adjacent ink dots that leads to non-uniform densities; an earlier placed dot should be held in place in the layer without "bleeding" into overlapping and latter placed dots; the layer should exhibit the ability to absorb high concentrations of ink so that the applied liquid ink does not run, i.e., there is no "ink-run off"; a short ink-drying time, and a minimum of haze. To meet these requirements, the ink-receptive layers of the prior art have been prepared from a wide variety of materials. One class of materials that has been described for use in ink-receptive layers of transparent image-recording elements is the class of vinyl pyrrolidone polymers. Typical patents are U.S. Pat. No. 4,741,969, issued May 3, 1988, which describes a transparent image-recording element having an ink-receptive layer formed from a mixture of a photopolymerizable, double-bonded anionic synthetic resin and another polymer such as a homo- or copolymer of N-vinyl pyrrolidone in which the mixture is cured to provide the ink-receptive layer, and U.S. Pat. No. 4,503,111, issued Mar. 5, 1985, which describes a transparent image-recording element for use in ink jet recording having an ink-receptive layer comprising a mixture of polyvinyl pyrrolidone and a compatible matrix-forming hydrophilic polymer such as gelatin or polyvinyl alcohol.
Unfortunately, transparent image-recording elements that have been described in the prior art and employ vinyl pyrrolidone polymers in ink-receptive layers have generally failed to meet the stringent requirements needed to provide a high quality image and this has significantly restricted their use.
In addition to the requirements already discussed, an important feature of a projection viewable image is the size and nature of the ink dots that form it. In general, a larger dot size (consistent with the image resolution required for a given system) provides higher image density and a more saturated color image and improves projection quality. A known method of increasing dot size involves applying liquid ink dots to a transparent image-receiving sheet, for example, HP PaintJet Film.TM. (commercially available from Hewlett Packard Company, Palo Alto, Calif.) using an ink jet printer. The sheet is dried for a short time, for example, 5 minutes, and inserted into a transparent plastic sleeve which protects the sheet and controls development of the dots. The sleeve compresses the dots and their size is increased to provide greater image density and color saturation upon projection of the image. Although this method is effective, it would be desirable to achieve appropriate dot size without the inconvenience of handling a separate sleeve.
In recently issued U.S. Pat. No. 4,903,041, issued Feb. 20, 1990, there is disclosed such a means. U.S. Pat. No. 4,903,041 discloses a transparent image-recording element adapted for use in a printing process in which liquid ink dots are applied to an ink-receptive layer such as an ink jet printing process where liquid ink dots are applied to an ink-receptive layer that contains a vinyl pyrrolidone polymer and particles of a certain polyester, namely, poly(cyclohexylenedimethylene-co-xylylene terephthalate-co-malonate-co-sodioiminobis(sulfonylbenzoate)), dispersed in the vinyl pyrrolidone to control ink dot size and to provide a high quality projection viewable image. By varying the concentration of polyester in the ink-receptive layers of the transparent ink-receiving elements disclosed and claimed in U.S. Pat. No. 4,903,041, there is provided a simple and expedient means for increasing the ink-dot size of the ink dots deposited on the ink-receptive layers disclosed and claimed therein. This results in an increase in the optical density of the ink-filled image areas formed on the surface of the ink-receptive layers of the transparent image recording elements disclosed and claimed in U.S. Pat. No. 4,903,041. However, while such elements constitute a significant advancement in the art by providing transparent image-recording elements which are adapted for use in printing processes where liquid ink dots are applied to an ink-receptive layer in which the ink dot size can be easily controlled to form ink-filled image areas having enhanced optical densities, it is desired that even further improvements in this regard be made.